A study on an organic electroluminescent (EL) device (hereinafter, simply referred to as ‘organic EL device’) has continued from the start point of observing an organic thin film light emission by Bernanose in the 1950s to blue electric light emission using an anthracene single crystal in 1965, and an organic EL device having a lamination structure, which is divided into functional layers of a hole layer and a light-emitting layer, was proposed by Tang in 1987. Thereafter, the organic EL device has been developed in the form of introducing a characteristic organic material layer into a device in order to enhance efficiency and a service life of the organic EL device, and the development has also been led to the development of specialized materials used therein.
In the organic electroluminescent device, when voltage is applied between two electrodes, holes are injected into the organic material layer at the anode and electrons are injected into the organic material layer at the cathode. When the injected holes and electrons meet each other, an exciton is formed, and when the exciton falls down to a bottom state, light is emitted. Materials used as the organic material layer may be classified into a light-emitting material, a hole injection material, a hole transporting material, an electron transporting material, an electron injection material, and the like according to the function.

Meanwhile, in order to achieve practical application of the organic electroluminescent device and enhance characteristics thereof, the device needs to be formed of an organic material layer having the multi-layered structure as described above, and a material for the device, particularly a hole transporting material, needs to have thermally and electrically stable characteristics. This is because when voltage is applied to an organic electroluminescent device, heat is generated from the device, and molecules having low thermal stability are rearranged due to low crystal stability, and as a result, there occurs a local crystallization, and thus there exists an inhomogeneous portion, and an electric field is concentrated on the inhomogeneous portion, thereby degrading and destroying the device.
In consideration of these points, m-MTDATA [4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)-triphenylamine], 2-TNATA [4,4′,4″-tris(N-(naphthylen-2-yl)-N-phenylamino)-triphenylamine], TPD [N,N′-diphenyl-N,N′-di(3-methylphenyl)-4,4′-diaminobiphenyl] and NPB [N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine], and the like were used in the related art as a hole transporting material.
However, since m-MTDATA and 2-TNATA have a low glass transition temperature (Tg) of about 78° C. and about 108° C., respectively, and many problems occur in the process of mass production, there has been a problem in implementing a full color. Meanwhile, TPD and NPB also have a low glass transition temperature (Tg) of about 60° C. and about 96° C., respectively, thereby causing deterioration in the service life of the device like m-MTDATA and 2-TNATA.
Therefore, there is a need for the development of a new hole transporting material which may increase thermal stability and has excellent hole transport capabilities, and thus may enhance the light-emitting efficiency and power efficiency of the organic electroluminescent device.